Humidity sensor

ABSTRACT

The operation of relative humidity sensors made from cobalt oxide on a non-conductive ceramic substrate is improved by heating the sensor for a short period to a high temperature in a reducing atmosphere. This reduces the specific resistance of the device. With a lower resistivity, it is possible to reduce the size of the sensor to the point where it can be included along with a semiconductor device in standard hermetic packages. This makes possible the continuous, on-line monitoring of hermeticity for the lift of the circuit without the necessity of applying a load to the circuit.

United States Patent 91 Frazee et al.

[4 1 Sept. 16, 1975 HUMIDITY SENSOR [75] Inventors: Lawrence E. Frazee, South Huntington, N.Y.; Anthony V. Fraioli, Essex Fells, NJ.

[73] Assignee: Plessey Incorporated, Melville, NY.

[22] Filed: Oct. 31, 1974 [21] Appl. No.: 519,604

Related US. Application Data [62] Division of Ser. No. 443,436, Feb. 19, 1974.

[52] US. Cl. 338/35; 29/610; 29/621 [51] Int. Cl. H01C 13/00 [58] Field or Search '338/35; 29/610, -6l9, 621;

340/235; 236/244 E; ZOO/61.06

[56] References Cited UNITED STATES PATENTS Delaney et al 338/35 Primary Examiner-C. L. Albritton Attorney, Agent, or FirmJames J. Burke, ll

[ ABSTRACT The operation of relative humidity sensors made from cobalt oxide on a non-conductive ceramic substrate is improved by heating the sensor for a short period to a high temperature in a reducing atmosphere. This reduces the specific resistance of the device. With a lower resistivity, it is possible-to reduce the size of the sensor to the point where it can be included along with a semiconductor device in standard hermetic packages. This makes possible the continuous, on-line monitoring of hermeticity for the lift of the circuit without the necessity of applying a load to the circuit.

3 Claims, 3 Drawing Figures "This compound is stable up to its istibovt: 1800C.

1 HUMIDITY SENSOR RELATED APPLICATIONS BACKGROUND OF THE INVENTlON sensors or hygrometers are well known. The startingmaterial is a cobalt oxide powder. As pure CoO powder is very expensive, the starting material is generally a mixture of C with some C0 0 but the latter compound dissociates at about 900C. so the-completed sensor will be essentially CoO, the cobaltous oxide. melting point, which -The finely divided powder, preferably minus325 mesh, is mixed with an inertliquid vehicle and viscosifiers 'to' form a screeiI-printable paste. A thin layer of the dielectric, high-temperature resistant substrate, typically a high-alumina ceramic. The screened pattern is then dried and fired in air at a temperature in the range of l350C. to 1550C. Electrodescan be preformed on the substrate, co-fired with the paste, or applied in a subsequent operation. The latter is the more common approach, as, it is generally desired to have the electrodes in a rather elaborate, interdigitated pattern on the top surface. Conductive inks orpastes (platinumgold, palladium-gold, etc.) are used in the conventional manner. w

- Before such a sensor can be put to use, it must beaccurately calibrated to determine the change in electrical resistance with relative humidity.

The-very high specific resistivity of cobalt oxide, which is ohms per square or higher, requires, that humidity sensors made therefrom be relatively large in 'ty'-se'nsit'ive"'to' a greater or lesser degree. As a result,

specifications" on hermeticity for packages are most stringent. But,'while technology for producing hermetic packages is well developed and tests therefor standarized, the fact that a package is hermetic initially 'says nothing about whether it will remain so after months or years of service, often under severe conditions of shock and vibration. Further, the failure of a seal in service can now be detected only by a malfunction or failure of the circuit.

PRIOR ART The production of cobalt oxide hygrometers is disclosed by Delaney et al. in US. Pat. No. 3,345,596.

The two patents of Nicholas, us. Pat. No. 3,703,697

and No.- 3,715,702, disclose theuse of humectants to increase water absorption on hygrometer surfaces of this same type, In one instance the oxide is converted I in part to the oxychlon'de, and in the other, a coating paste is then screen-printed in a desired pattern onto a 1 provide a-semiconductor package with tegral humiditysensor.

face.

' of lithium chloride or polyethylene glycol is provided.

Blythe et al.', US. Pat. No. 3,105,214, disclose the use of a vapor-permeable ion-selective membrane on the sensor surface. Thiswill swell up in a humid environment andtransport water but not ions to the sensor sur- OBJECT OF THE INVENTION A general object of the present invention is to provide improved humidity sensors of the cobalt oxide type.

Another object of the present invention is to provide a cobalt oxide humidity sensor which is smaller than prior art sensors. I I

A further object of the present invention is to provide a cobalt oxide humidity sensor within'a hermetically sealed semiconductor package.

A still further object of the present invention is to provide acobalt oxide humidity sensor having a'lower resistivity than-prior art'sensors of the same general y I Yet another object of the present invention is to provide a cobalt oxide humidity sensor very sensitive in the low-ppm range.

A still further object of the present invention is to a built-in or in- Various'other objects and advantages of the invention' will become clear from'the following description of embodiments thereof, and the novel features will be particularly pointed out in connection with the appended claims.

THE DRAWINGS to the accompa- DESCRIPTION OF EMBODIMENTS The present invention comprises, in essence, lowering the specific-resistivity ofthe cobalt oxide film by heating the previously fired film to a high temperature in a reducingatmosphere for a brief period. The atmosphere is preferably hydrogen. This treatment has been observed to reduce the resistivity of the film by l to I 1.25 orders of magnitude. Thus, an air-fired-CoO film with a resistivity of 3.76 X 10 ohms/square hasa resistivity of 6.3 X 10 ohms per square after firing in hydrogen at 1500C. for 15 minutes.

- The reasons why this treatment reduces resistivity are unclear, just as the mechanism of conduction in cobalt oxide is unclear. It is possible that the reducing gas attacks the C00 crystallite boundaries preferentially and exposes more surface with a higher surface energy. It

It is believed that the prior-art (Delaney etai.) teaching of firing the cobalt oxide for a brief period only and at a high temperature (l500C.) is designed to provide a large number of very small CoO'crystals and prevent crystal growth. The hydrogen treatment of the present invention may significantly increase the populationdensity of exposed crystal edges and corners. Because of the interrupted lattice periodicity at these locations, and resultant dangling or broken bonds, these sites should be preferred locations for gas adsorptiondesorpseveral reasons, the most important of. which is thatv it permits a substantial reduction in the size of the humistor. Thus, heretofore a typical humistor had to be mounted on a linch: square substrate, to allow for device and conductive geometry that wouldgive asuitable output. In accordance with the present invention, humistors of the same resistance are produced-on 0.20

by 0.20 by 0.040 inch substrates with the.same,( l.5

volt) outputs and improved linearity .of; response.

This permits humsitors of the present invention to be qused for'the first time, as hermeticity detectors within semiconductive device packages. As noted hereinabove, stringent hermeticity requirements are imposed in both military and commercial electronics specifications, because humidity has a disastrous effect on the operation of both active and passive microcircuit elements. Heretofore, it has not been-possible to detect hermeticity failures, i.e. leaks, except through circuit mal-functions. With the miniaturized humiditysensors I? produced in, accordance with the present invention, it

becomes possible to include the device on the substrate within the can or package. In operation with a nominal bias, it will be essentially non-conductive. If the package fails and even a few ppm of moisture are encountered, the sensor will be rendered sufficiently conductive to trigger an alarm. Thus, with the present invention it is possible to continuously monitor hermeticity without waiting for circuit .damage, mal-function orfailure, and hermeticity monitoring is carried out without loading the main circuit. More particularly, humistors of'the present invention may have a total wet-todry resistance change of as much as six or seven orders of magnitude. This very great change is believed toaccount for the substantial sensitivity in the very low ppm (high resistivity) range.

Humistors made in accordance with the present invention resemble prior art devices except insofar as the hydrogen firing tends to change the normally rather glossy CoO surface to a more matte-like appearance.

- As shown in FIG. 1, a ceramic substrate has a layer of cobalt oxide 12 screened and firedover its entire surface (or less than the entire surface, as desired).

. First'and second electrode patterns 14, 16 which terminate in bonding pads 18,; 20 are then screened and fired. After firing in a reducing atmosphere (which may occur before or afteraapplication of electrodes) the de vice is complete.

FIG. 2 illustrates. a humidity sensor of the present invention incorporated into a standard dual in-line 14-' lead package. The substrate 22 first has a cobalt oxide area 24 screened and fired thereon. Thereafter, the integrated circuit leads 26, the humistor electrodes 28, and leads therefore are all screened and fired simul- .taneously. The entire unit is then fired in hydrogen,

since this will noteffect the metallic patterns. Installation of the integrated circuit 32 or other semiconductive device, sealing and testing follow conventional procedures, except that further exposure to reducing atmospheres should be avoided.

. Understanding of the invention will be facilitated by 7 reference to the following specific example, which is to be construed as illustrativeonly and not in a limiting sense. I 7

EXAMPLE in humistors .(or resistors) of the general type tiescribed, calculation of resistivity in ohms/squre is more complex than for simple thick fiim geometries, and depends on the space between respective interdigitated electrodes and their length. With reference to FIG. 3, the number of squares is calculated according to the following formula:

In Equation 1, N is thetotal number of conductor lines,

-so 'N-l is the number of resistor lines. The factor L/W is-multi'plied' by the reciprocal of N-l because, while L. is constant throughoutthe pattern, W is lengthened by each pair of conductors, and it is this which must be multiplied by N-l. As will be apparent from the data hereinbelow,*thisproduces resistors with very small numbers of' squares.

Cobalt oxide paste was screened and fired onto one inch substrates in a' 950 mil square pattern and fired.

One group of substrates was fired in hydrogen and all substrates had identical conductives applied, wherein that were not hydrogen fired had resistances of 3.2 X

.10 0. and those that were so treated measured L9 X 1 0 0.. In terms of ohms/square, these figures convert to 4.26 X 109 and 2.53 X 10*, respectively.

' The same paste was screened in 200 mil square patterns and, again one group was fired in hydrogen and another was, not. Conductives were applied in a pattern where The number of squares in this pattern is 1.19047 10?? During processing, film thickness, firing parametersietc. were all closely controlled so that the films were comparable except for size and conductive geometry. On these units, at 100% R.H., resistance of the non-hydrogen fired units was 5.07 X and those that were so treated measured 3.01 X 10 0. Resistance of 200 mil square humistors treated in accordance with the present invention is thus seen to be substantially the same as 950 mil square units that were not so treated (i.e. 3.2 X 10 0).

l-lumistors are calibrated by plotting log R vs. relative humidity over the entire humidity range. A problem with known humistors has been an asymptotic resistance change (or other anomalies) as RH. approaches 0%. An advantage of humistors made in accordance with the present invention is a substantially linear response over the entire RH. range. This greatly simplifies required circuitry and makes the devices useful as hermeticity detectors, where RH. must be measured in parts-per-million. The reason why the reducing-gas treatment brings about this desirable improvement is not understood.

It will be appreciated by those skilled in the art that FIG. 2- is illustrative only and is not to be construed in a limiting sense, since many variations are possible. Thus, a discrete humistor as shown in FIG. 1 could be incorporated into a semiconductor package rather than having the package manufactured with an integral device. The humistor could be wired into the main circuit if that condition could be tolerated. Different package designs offer varying opportunities for placement of the sensor. In packages having glass-sealed ceramic lids for example, it would be possible to have the sensor printed on the under-side of the lid. Discrete or integral sensors could be incorporated into header cans. ln laminated packages having leads on a raised s'tep section surrounding the device in a central cavity, the sensor could be placed on either level.

It will be further appreciated that the small size of humistors of the present invention permits their incorporation in a variety of other devices where herrneticity is important. Image intensifier tubes or other electronic tubes are one example. Optical systems where hermeticity is required to avoid condensation on optical elements is another example.

Various other changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.

What is claimed is:

1. A humidity sensor comprising:

a high-temperature resistant, dielectric substrate;

a double-fired layer of cobalt oxide on a major surface of said substrate, said double-fired layer having a lower than normal resistivity as a result of a second firing in a reducing atmosphere at about 1500C. for a brief period; and

a fired-0n pair of electrodes in contact with said double-fired layer.

2. The humidity sensor as claimed in claim 1, wherein the resistivity of said double-fired layer is at least one order of magnitude less as a result of said second firing.

3. The humidity sensor as claimed in claim 1, wherein said electrodes are in the form of closely-spaced, inter- 

1. A HUMIDITY SENSOR COMPRISING: A HIGH-TEMPERATURE RESISTANT, DIELECTRIC SUBSTRATE, A DOUBLE-FIRED LAYER OF COBALT OXIDE ON A MAJOR SURFACE OF SAID SUBSTRATE, SAID DOUBLE-FIRED LAYER HAVING A LOWER THAN NORMAL RESISTIVITY AS A RESULT OF A SECOND FIRING IN A REDUCING ATMOSPHERE AT ABOUT 1500*C FOR A BRIEF PERIOD, AND A FIRED-ON PAIR OF ELECTRODES IN CONTACT WITH SAID DOUBLEFIRED LAYER.
 2. The humidity sensor as claimed in claim 1, wherein the resistivity of said double-fired layer is at least one order of magnitude less as a result of said second firing.
 3. The humidity sensor as claimed in claim 1, wherein said electrodes are in the form of closely-spaced, interdigitated fingers. 